Add solution to problem four

four.adb

@@ -0,0 +1,75 @@
+--
+--  Project Euler problem #4
+--
+--  A palindromic number reads the same both ways. The largest palindrome made
+--  from the product of two 2-digit numbers is 9009 = 91  99.
+--
+--  Find the largest palindrome made from the product of two 3-digit numbers.
+--
+
+with Ada.Strings.Fixed,
+     Ada.Text_IO;
+
+use Ada.Strings.Fixed,
+    Ada.Text_IO;
+
+procedure Four is
+    Ceiling : constant Natural := 999;
+
+    function Reversed_String (Forward : in String) return String is
+        Result : String (Forward'Range);
+    begin
+        for Index in Result'Range loop
+            Result (Index) := Forward ((Forward'Length + 1) - Index);
+        end loop;
+        return Result;
+    end Reversed_String;
+
+    function Is_Palindrome (Input : in Natural) return Boolean is
+        Raw_Input_String : constant String := Natural'Image (Input);
+        -- Trim the leading space off the stringifyed input
+        Input_String : constant String := Raw_Input_String (2 .. Raw_Input_String'Last);
+        Length : constant Natural := Input_String'Length;
+    begin
+        if (Length mod 2) /= 0 then
+            return false;
+        end if;
+
+        declare
+            First_Half : constant String := Head (Input_String, (Length / 2));
+            Second_Half : constant String := Reversed_String (Tail (Input_String, (Length / 2)));
+        begin
+            if First_Half = Second_Half then
+                return true;
+            end if;
+        end;
+
+        return false;
+    end Is_Palindrome;
+
+    Highest_Palindrome : Natural := 0;
+begin
+    for Offset in 0 .. Ceiling loop
+        declare
+            N : constant Natural := Ceiling - Offset;
+        begin
+            for Sub_Offset in 0 .. Ceiling loop
+                declare
+                    J : constant Natural := Ceiling - Sub_Offset;
+                    Product : constant Natural := J * N;
+                    Bailout : constant Boolean := Is_Palindrome (Product);
+                begin
+                    if Bailout then
+                        if Product > Highest_Palindrome then
+                            Highest_Palindrome := Product;
+                        end if;
+                    end if;
+                end;
+            end loop;
+        end;
+    end loop;
+
+    New_Line;
+    Put_Line ("Highest:" & Natural'Image(Highest_Palindrome));
+end Four;
+